The present invention relates to a process for an integrated system for the evaporation of spent liquors and recovery of alcohols in cellulose pulping processes using cooking liquors based on alcohols or corresponding cooking liquors, e.g. organic solvent pulping. In such processes the cooking liquors contain alcohols as active substances, such as methanol, ethanol, or other liquids having a lower boiling point than water.
In cellulose solvent pulping processes, the chips are first steamed. A prehydrolizing step may then follow. The cooking is carried out in two stages. In the first stage, the chips are cooked with a solution of chemicals containing about half methanol and half water. The cooking is effected at a temperature of about 190.degree. C. and takes from 20 to 50 minutes. In the second stage, caustic soda having a concentration of 18 to 22 percent is added. The temperature is maintained between 165.degree. and 175.degree. C., the second stage being practiced about 60 minutes.
After cooking, or partly integrated with the cooking stage, washing of the pulp begins. The purpose of the wash is to wash away from the pulp substances which have been dissolved during cooking, such as lignin, and initiate the recovery of cooking chemicals. The spent washing liquid contains methanol, partly deriving from the supplied cooking liquid, partly produced during the cooking. The percentage by weight of methanol is on the order of about 17 to 20 percent. The spent liquor also contains about 10 to 15 percent dry matter. The spent liquor is usually evaporated to a dry matter content of about 55 to 65 percent so that the liquor --also called thick liquor--can be combusted in a boiler and chemicals can be recovered from dissolved organic substances, recovering heat at the same time.
When the spent liquor arrives at the evaporator system, besides having a high temperature, it is also under a high pressure and contains volatile substances to such a degree that it is classified as being easily inflammable and explosive. All this renders intermediate storing of the spent liquor before and during the evaporation expensive and difficult. Storing must take place in a pressurized vessel, or the liquor must be cooled to a temperature lower than 50.degree. C. before being stored.
According to the present invention, evaporation and the recovery of chemicals from spent solvent pulping liquor can be carried out so that no buffer container or intermediate storage is needed. When the spent liquor arrives at the evaporator system, which usually is a multistage evaporator, water is at first evaporated from the liquor and the content of volatiles is reduced by expansion in several steps, thereby making evaporation possible. A high content of volatiles, such as methanol, lowers the condensing temperature considerably and also impairs the heat transfer in other ways, which renders the conventional cascade evaporating method less economical. These problems have been solved by the present invention. The heat transfer can be improved when divided heat surfaces are used for evaporation, which makes ample venting and an early separation of volatiles possible. For that reason, intermediate storage of the spent liquor is not necessary. Heating condensation is carried out in each evaporator apparatus, at least before the final thickener, on several heat surfaces separated from each other. The heat surfaces in the evaporation effects can comprise tubes or plates.
A part of the heat surface in each evaporator effect can condense liquor vapor from the preceding effect, and a separate part of the heat surface in the same effect can condense expansion liquor vapor. The liquor vapor contains less volatiles than the expansion liquor vapor. As these vapors, according to the invention, are not mixed with each other prior to their use as heating medium in the next step, it is possible to utilize a higher condensing temperature in an efficient way. It is then also possible to treat condensates containing different amounts of volatiles separately.
Additional advantages are, according to the invention, obtained by injecting pure water, vapor condensate, or vapor (steam) at the warmer side of the evaporator in order to increase the condensing temperature, which in turn increases the available useful temperature difference. Addition of water to the vapor brings about a new condition of equilibrium between vapor and liquid, in which the concentration of volatiles, such as methanol, in the vapor, is lower and the condensing temperature of the vapor is higher. Water is preferably added to methanol-rich vapors, such as expansion liquor vapors, which are produced in expansion tanks before the evaporation units.
On the liquor side of the evaporation apparatus, the liquor is circulated up to distribution means--one or several--which are either common or divided for the divided heat surfaces, and the liquor vapor is evaporated in a vapor compartment which is not divided.